As integrated circuit features are scaled down and density increases, material properties such as resistivity, which influences observed resistance, exhibit relatively more pronounced effects. In addition, the reliability of integrated circuits is affected by a number of stresses that increase as feature size drops and density increases. These stresses include electrical, thermal, mechanical and environmental stress. Electromigration is an example of phenomena that reduce semiconductor reliability, lead to interconnect failure, and become relatively more prominent as feature size decreases, particularly below 50 nm, and power density increases. Electromigration is understood as the transport of material due to movement of ions in a conductor. Electromigration may result in the formation of hillocks or voids within the interconnect, and eventually lead to its failure.
To reduce electromigration, and other stress induced failures, refractory metals have been used in interconnect fabrication. However, refractory metals exhibit increased resistivity and, therefore, increased resistance and resistive-capacitance (RC) delay. To further reduce electromigration, and other stress induced failures, diffusion barriers have been deposited on the side and bottom walls of openings in interlayer dielectrics containing the interconnects. Diffusion barriers are understood to typically occupy a small fraction of the cross-sectional area of the interconnect line. The portion of the interconnect exposed at the surface of a given dielectric layer is coated with an insulator such as silicon nitride. However, the use of an insulative capping layer may adversely affect line performance by increasing capacitive coupling.
Therefore, as feature sizes continue to decrease, room remains for the improvement in the design of interconnects with, in some instances, an emphasis on both interconnect RC and resistance to various stresses, such as those resulting in electromigration and thermomechanical failures.